"Digital currency" is a wide term. At one extreme it can be a token that can be used anonymously and without the need for a bank in the same way coins and bank notes are today: As long as people are satisfied that they are genuine they are happy to accept them as payment.

The point being is that what is referred to as "digital currency" is not what we have now (otherwise the BoE and others would not be studying feasibility and costs/benefits). In that sense currency is not digital at the moment. Bank ledgers and transfer instructions are.